1) Field of the Invention
The present invention relates to a technology for generating a high-frequency pulse current in order to drive a laser diode or the like.
2) Description of the Related Art
FIG. 19 is a circuit diagram that shows a configuration example of a conventional pulse current generation circuit formed of an integrated circuit with MOS transistors. However, a configuration example of bipolar transistors is not shown.
As shown in FIG. 19, this pulse current generation circuit includes NMOS transistors 101 and 102, and a bias voltage source 103. A laser diode 106 is connected to an IC output terminal 104 and an external power supply 105. A snubber circuit 107 is connected in parallel with the laser diode (LD) 106.
A bias voltage of a positive polarity is input from the bias voltage source 103 to the NMOS transistor 101 at its gate electrode. The NMOS transistor 101 is connected at its drain electrode to the IC output terminal 104 and connected at its source electrode to the NMOS transistor 102 at its drain electrode. The NMOS transistor 102 is connected at its source electrode to ground (GND). An input pulse p having a predetermined pulse width is applied to the NMOS transistor 102 at its gate electrode.
In the configuration heretofore explained, the bias voltage of the positive polarity is applied from the bias voltage source 103 to the gate electrode of the NMOS transistor 101. Thus, the NMOS transistor 101 is in the ON state, and forms a constant current source that flows a preset constant current. When the input pulse p is applied to the gate electrode of the NMOS transistor 102 and the NMOS transistor 102 turns ON, the LCD 106 is connected at its cathode to ground (GND) via the NMOS transistors 101 and 102.
Accordingly, the LD 106 turns ON. A pulse current having a pulse width equivalent to that of the input pulse p flows to the ground (GND) via the LD 106 and the NMOS transistors 101 and 102. In other words, when the NMOS transistor 102 is turned ON by the input pulse p, an output pulse current having the pulse width is applied to the LD 106, and consequently the LD 106 is subjected to pulse driving.
If the slew rate becomes high in pulse light emission of the LD 106, then overshoots, undershoots, and ringings occur due to parasitic inductance components of an IC substrate. In the worst instance, the LD 106 is destroyed.
In the conventional art, therefore, there is adopted a method of connecting the snubber circuit 107, which is a filter made up of a series circuit consisting of a resistor and a capacitor, in parallel with the LD 106 as shown in FIG. 19 in order to reduce the overshoots, undershoots, and ringings.
In the method of externally providing the snubber circuit 107 in order to reduce the overshoots, undershoots, and ringings, however, the number of components increases and requires much labor to set constants of the snubber circuit 107. Due to addition of the snubber circuit 107, the slew rate is lowered. This results in a problem that the data transfer rate cannot be increased in application to communication or storage.
On the other hand, when a distributed parameter circuit can be supposed as in long external wiring, there is a method of effecting impedance matching and thereby reducing overshoots, undershoots, and ringings caused by reflection.
This method is used frequently when wiring impedance is not so high and a pulse voltage is output with low output impedance. It is difficult to effect matching in an IC because it is necessary to make the impedance of external wiring a certain constant value. In other words, the pulse generation type is comparatively high in output impedance, and consequently the impedance matching technique cannot be adopted.